1. Field of the Invention
The present invention relates to RFID couplers and, in particular, to UHF spatially selective couplers capable of selectively communicating with a targeted transponder from among of group of multiple adjacent transponders.
2. Description of Related Art
Radio frequency identification (RFID) transponders, either active or passive, are typically used with an RFID transceiver or similar device to communicate information from the transponders. In order to communicate, the transceiver exposes the transponder to a radio frequency (RF) electromagnetic field or signal. In the case of a passive transponder, the RF electromagnetic field energizes the transponder and thereby prompts the transponder to respond to the transceiver by modulating the field in a well-known technique called backscattering. In the case of an active transponder, the transponder may respond to the electromagnetic field by transmitting an independently powered reply signal to the transceiver.
Problems can occur when interrogating multiple adjacent transponders regardless on whether the transponders are passively or actively powered. For example, an interrogating electromagnetic signal may activate more than one transponder at a given time. This simultaneous activation of multiple transponders may lead to communication, i.e. read and write, errors because each of the multiple transponders may transmit reply signals to the transceiver at the same time.
Several anti-collision management techniques commercially exist for allowing near simultaneous communication between multiple transponders and a single transceiver while reducing communication errors. However, such anti-collision management techniques tend to increase system complexity, cost, and delay response. Furthermore, such techniques are often “blind” in that they cannot locate a given transponder or more specifically recognize the position of a transponder within the interrogating RF electromagnetic field. For example, in a printer-encoder device, the device would not know whether the transceiver was communicating with the transponder proximate to the printhead or not.
Another method of preventing multiple transponder activation is to electrically isolate transponders from one another. For example, devices or systems may employ an RF-shielded housing or anechoic chamber for shielding the adjacent transponders from the electromagnetic field. In various applications, transponders individually pass though a shielded housing for individualized exposure to an interrogating RF electromagnetic field. Unfortunately, RF-shielded housings add cost and complexity to a system. Furthermore, many devices are limited with regard to space or weight and, thus, cannot accommodate such shielded housings.
The challenge of avoiding multiple transponder activation may be especially acute in some applications. RF printer-encoders are one example. RF printer-encoders are devices capable of programming and printing a series or stream of transponders. The close proximity of the transponders, during processing, and the space, cost, and weight restrictions associated with such devices make multiple transponder activation problematic. Furthermore, the space, cost, and weight restrictions, among other factors, make anti-collision management techniques or shielding components for alleviating multiple transponder activation less than desirable.
In light of the foregoing it would be desirable to provide a RF system or device capable of interrogating individual transponders positioned among multiple adjacent transponders without the need for anti-collision management techniques or shielding components. Furthermore, it would be preferable to provide an RF communication system that adjusts to different transponder configurations and placements without increasing the broadcast range of the RF transceiver signal.